Extruded solid insulation based on a polyethylene, or a cross linked polyethylene (XLPE), has for almost 40 years been used for AC transmission and distribution cable insulation.
Therefore the possibility of the use of XLPE for DC cable insulation has been under investigation for many years. Cables with such insulations have the same advantage as the mass impregnated cable in that for DC transmission there are no restrictions on circuit length and they also have a potential for being operated at higher temperatures, thus offering a possibility to increase the transmission load.
However, it has not been possible to obtain the full potential of these materials for full size cables. It is believed that one of the main reasons being the development and accumulation of space charges in the dielectric when subjected to a DC field. Such space charges distort the stress distribution and persist for long periods because of the high resistivity of the polymers. Space charges in an insulation body do, when subjected to the forces of an electric DC-field, accumulate in a way that a polarized pattern similar to a capacitor is formed.
There are two basic types of space charge accumulation patterns, differing in the polarity of the space charge accumulation. The space charge accumulation results in a local increase at certain points of the actual electric field in relation to the field, which would be contemplated when considering the geometrical dimensions and dielectric characteristics of insulation. The increase noted in the actual field might be 5 or even 10 times the contemplated field.
Thus, the design field for cable insulation must include a safety factor taking account for this considerably higher field resulting in the use of thicker and/or more expensive materials in the cable insulation. The build up of the space charge accumulation is a slow process, and therefore this problem is accentuated when the polarity of the cable after being operated for a long period of time at same polarity is reversed. As a result of the reversal a capacitive field is superimposed on the field resulting from the space charge accumulation and the point of maximal field stress is moved from the interface and into the insulation. Attempts have been made to improve the situation by the use of additives to reduce the insulation resistance without seriously affecting other properties.
An extruded resin composition for AC cable insulation typically comprises a polyethylene resin as the base polymer complemented with various additives such as a peroxide cross linking agent, a scorch retarding agent and an anti-oxidant or a system of antioxidants. In the case of an extruded insulation the semi-conductive shields are also typically extruded and comprise a resin composition that in addition to the base polymer and an electrically conductive or semi-conductive filler comprises essentially the same type of additives. The various extruded layers in an insulated cable in general are often based on a polyethylene resin. Polyethylene resin means generally and in this application a resin based on polyethylene or a copolymer of ethylene, wherein the ethylene monomer constitutes a major part of the mass.
Thus polyethylene resins may be composed of ethylene and one or more monomers which are co-polymerisable with ethylene. Low density polyethylene (LDPE) is today the predominant insulating base material for AC cables. To improve the physical properties of the extruded insulation and its capability to withstand degradation and decomposition under the influence of the conditions prevailing under production, shipment, laying, and use of such a cable the polyethylene based composition typically comprises additives such as stabilizing additives, e.g. antioxidants, electron scavengers to counteract decomposition due to oxidation, radiation etc., lubricating additives, e.g. stearic acid, to increase processability, additives for increased capability to withstand electrical stress, e.g. an increased water tree resistance, e.g. polyethylene glycol, silicones etc., and cross-linking agents such as peroxides, which decompose upon heating into free radicals and initiate cross-linking of the polyethylene resin, sometimes used in combination with unsaturated compounds having the ability to enhance the cross-linking density, scorch retarders to avoid premature cross-linking.
The number of various additives is large and the possible combinations thereof are essentially unlimited. When selecting an additive or a combination or group of additives the aim is that one or more properties shall be improved while others shall be maintained or if possible also improved. However, in reality it is always next to impossible to forecast all possible side effects of a change in the system of additives. In other cases the improvements sought for are of such dignity that some minor negative have to be accepted, although there is always an aim to minimize such negative effects.
Although some disadvantages with the use of an XLPE composition have been known for a long time its advantages (e.g. its ability to prevent scorch, i.e., premature cross linking) have outweighed these drawbacks. Furthermore it is well known that this type of XLPE composition exhibits a strong tendency to form space charges under DC electric fields, thus making it unusable in insulation systems for DC cables. However, it is also known that extended degassing, i.e., exposing the cross linked cable insulation to high temperatures for long periods of time, will result in a decreased tendency to space charge accumulation under DC voltage stress. It is generally believed that the heat treatment removes the peroxide decomposition products, such as acetophenone and cumyl alcohol, from the insulation whereby the space charge accumulation is reduced. Degassing is a time-consuming batch-process comparable with impregnation of paper insulations and thus as costly. Therefore it is advantageous if the need for degassing is removed. Most known cross-linked polyethylene compositions used as extruded insulation in AC-cable exhibit a tendency for space charge accumulation which renders them unsuitable for use in insulation systems for DC cables.